Injection molding is used to manufacture a variety of plastic products. Molds used in these processes typically have several sections that when put together define a cavity in which molten plastic resin is injected.
To ensure that the molten plastic resin fills all of the details in the mold cavity, the molten plastic resin is preferably injected into the mold under pressure. The pressures that the molds are subjected to can be extreme and, as such, the mold components are often massive to support such pressures.
The resin pathways, “hot runners,” and the nozzles used to inject the molten plastic resin into the mold cavities have ancillary heating to properly maintain the molten plastic resin at a desired temperature. Often other areas of the molds need ancillary heat for controlling molding parameters, for example, controlling the rate of curing or hardening of the molten plastic. Johnson et al., U.S. Pat. No. 6,325,615, and Gellert, U.S. Pat. No. 5,148,594, both relate to systems for heating regions of molds. Because of the relatively hot temperatures and demanding environment at which the heating elements operate, they are subjected to degradation over extended use.
The heating elements are often placed in a meandering channel formed in the mold or mold plate where heat is desired. The heater will typically have a heat generation of approximately 50 watts per inch and the channel will typically be 0.300 to 0.500 inches in diameter. It is imperative that there be good thermal contact between the heater and the channel sidewall surfaces to provide the necessary heating to the mold components as well as to maximize the life of the heater. Ceramic paste or other material may then be utilized to fill the channel. Due to the diameter of these heaters they in the past have not been readily bendable. Attempts to manually bend conventional tubular heaters will generally result in kinks which ruins the heater. Conventionally, the heaters will be bent at the manufacturer or distributor using suitable jigs and powered equipment to the shape of the channel and then shipped to the end user. This adds problems if the bending is not totally accurate, increases the price of the heaters, and causes delays when a heater needs to be replaced. Ideally, the tubular heaters should be manually bendable for placement in the heaters by the end users. They could then be kept in stock and used as needed. Several manually bendable tubular heaters are illustrated in the prior art but they have various drawbacks.
Schmidt, U.S. Pat. No. 5,225,662, discloses a flexible heating element in which the heater core is covered with a plurality of beads. When the beads are placed in an adjacent relationship, the beads overlap each other to thereby protect the heater core from damage. This configuration does not present the possibility of a hermetically sealed tubular heater and can be difficult to manufacture.
Schwarzkopf, U.S. Pat. No. 6,250,911, describes an electrical heater for a mold in an injection molding machine. This patent indicates that the outer casing is formed from a highly ductile metal. The heating element and the insulating material that extends between the heating element and the casing are also flexible. This configuration for the electrical heater is stated to permit the heater to be bent by hand.
Schwarzkopf, U.S. Pat. No. 6,408,503 discloses a method of making an injection mold heating element. The method includes filling a region between a heating wire and an outer casing with a compressible insulating material. The casing is then radially inwardly compressed to form annular grooves.
Although the above heaters and methods of manufacturing them may work in certain applications, such designs may be improved upon to provide more heater to channel wall contact, better containment of the heater element and insulative material, easier and less expensive manufacture, manual or improved manual bendability, capability of bending tighter radii, and better reliability.